3 Title: MAC protocol for wireless sensor networks Theme: Robust communication Project period: 6th semester, February - May 2007 Project group: Communication Systems, group 652 Participants: Giuseppe Calí Ileana Ghizd vescu Anders Grauballe Mikkel Gade Jensen Fabio Pozzo Supervisors: Tatiana Kozlova Madsen Frank Fitzek Number of prints: 7 Number of pages: 95 Number of appendixes and character: 1 pcs. CD-ROM Finished: May 30th, 2007 Synopsis: Department of Electronic Systems Communication Systems Fredrik Bajers Vej 7C Telephone Fax Wireless sensor networks consist of spatially distributed autonomous small devices, often called "motes", which cooperatively monitor, collect and exchange data from the surrounding environment. Aalborg University has developed a mote which integrates a Bluetooth module and a low cost Industrial, Scientic and Medical (ISM) band module which makes possible to establish a multi-hop connection between the motes. The purpose of this project is to design and implement a Medium Access Control (MAC) protocol for the ISM module of the mote platform, proving a solution to avoid collisions between packets during transmission. A collision avoidance scheme with acknowledgements and carrier sensing has been designed and implemented to minimize data loss and duplication. This is also known as Request-To-Send (RTS) / Clear-To-Send (CTS) medium reservation mechanism. To test the implementation of this protocol, a mobile phone application is developed which allows a user to exchange text, image and audio les through the mote network. The acceptance test concludes that the implementation is robust and works as stated in the requirements specication except for one requirement regarding maximum transmission range. This is however due to the antenna calibration and is not software related. The written material in the report is public available.

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5 Preface This project has been carried out by project group 652, Communication Systems on 6th semester at Aalborg University, spring The focus group for this report is people with an interest in wireless sensor networks, MAC protocols, mobile development and the idea of communication between sensors and mobile phones. In this report gures, pictures and tables are labeled with chapter and gure number for easy reference, e.g. 4.2 for second gure in chapter 4. References for literature are shown as e.g. [10]. A CD is attached to the back cover of the report containing the following: The report in PDF format. The source code for the programs in C and Python. There are several people we would like to thank for their involvement and for helping us completing this project. Therefore, we would like to express our gratitude to our supervisors Tatiana Kozlova Madsen and Frank F. Fitzek who have guided us in our work. Their advice, idea and support, throughout the project has been very helpful. The same holds true for Ben Krøyer who designed and built the sensors. Furthermore, we are thankful to Stephan Rein and Daniel Gühne from TU Berlin for code examples and programming support. Finally, our thanks go to Gian Paolo Perrucci, for the Python programming support.

10 Chapter 1 Introduction The demand for gaining information about the surrounding environment is growing and throughout the last decades devices has been invented for this purpose. One of the new topics in this research area is wireless sensor networks that can provide this information in a fast and easy way. Wireless sensor networks consists of several small devices referred to as motes that in a cooperative way can collect and exchange information from the surrounding environment. They can be used to monitor e.g. temperature, pressure, motion etc. or they can be utilized to create a small wireless communication network. Wireless sensor networks benets from other traditional networks in price and size. A qualitative comparison of wireless sensor networks, wireless ad-hoc networks and wired Networks can be seen in Table 1.1. The table can help deciding which kind of network to deploy in a given scenario.

12 FIRST AAU MOTE Figure 1.1: The gure shows a possible setup of motes for an application. A mote (red square) measures the environment and relays the result through the network to a gateway (mobile phone) for viewing or further forwarding. Other possible applications for a wireless sensor network are briey described in the following: Smoke detection is of great importance in various environments including private homes. Such systems of cooperating smoke detectors already exist. When a possible re is detected by one sensor it can alert the others setting o the alarm in every one, or the message can be conveyed through the network to a main control station. Temporary setups of communication or security systems can be a simple solution in situations of urban warfare or natural disasters. Such events often result in broken cellular networks or potential risk for aid workers. Wireless sensor networks is fast and easy to deploy for these purposes Parking of a car in a small space or driving reverse with accuracy can be improved by attaching ultra sonic distance sensors to the car. By continuous measurements transmitted to a receiver (mobile phone) the driver is able to move the car closer to other cars or objects. This could be a cheap alternative to installing parking cameras in the car. 1.1 First AAU mote Motes are in theory very cheap but the rst prototypes are having a price of approximately 100 US dollars. Aalborg University have developed its rst prototype of such a mote which can measure distances by an ultra sonic ranger and broadcast this measurement via Bluetooth. This mote can be seen in Figure 1.2. The goal for AAU is to develop generic motes capable of adapting to a wide set of applications.

13 1.2. SECOND AAU MOTE 11 Figure 1.2: The gure is showing the distance sensor mote developed by Aalborg University Each mote contains a microprocessor, communication module, sensor module and power supply. All of this is contained in a box typically about the same size as a mobile phone. The rst motes developed at Aalborg university does only support communication via Bluetooth. The Bluetooth module is the most costly part of this mote, therefore it would be ideal also to integrate a low cost ISM band (industrial, medical and scientic) 433 MHz transceiver module. The Bluetooth implementation of the mote does only support a one-hop connection so the idea is to implement the ISM band module so the motes are able to make multi-hop connections as it is seen in Figure Second AAU mote A new mote is being developed at Aalborg University and can be seen in Figure 1.3. As seen the mote consists of two dierent boards stacked on each other which gives the exibility of adding more features in a later design. This new mote is the one being used in this project and the current one has the features described in the following subsections. See Appendix F for further information about hardware schematics.

14 SECOND AAU MOTE Figure 1.3: The gure is showing the second version of the mote developed by Aalborg University Main board Li-Poly battery interface which can be used for power supply to the mote Mini USB interface which can be used for both serial RS-232 interface to the mote and external power supply. It is also possible to charge the Li-Poly battery by changing the jumper settings on the board. dspic microprocessor for controlling the mote (further described in Section 1.2.3) 22.1 MHz oscillator as external clock source for the dspic ICD 2 debugger/programmer interface Wireless board Bluetooth module (Amber Wireless AMB2300) for wireless communication with mobile phones or PCs, connected with serial RS-232 connection to the microprocessor RF transceiver for communicating via the ISM band. (Further described in Section 1.2.4). Loop antenna for the RF transceiver

15 1.2. SECOND AAU MOTE Microprocessor The mote is controlled by a microprocessor from Microchip with the product name dspic33fj256gp710 referred to as dspic, which is a 16-bit Digital Signal Controller (DSC) based on the modied Harvard architecture. It has the following relevant features that can be utilized in this project: 256 KB Flash memory 30 KB RAM 85 programmable digital I/O pins, 100 pins in total Two (Universal Asynchronous Receiver Transmitter) UARTs Two (Serial Peripheral Interface) SPIs Nine 16-bit timers C compiler optimized instruction set, 83 instructions [3] One UART is used for RS-232 serial communication and is connected to the USB interface. The second is connected to the Bluetooth module RF transceiver To communicate in the unlicensed ISM band the RF transceiver (Nordic Semiconductor nrf905) referred to as nrf, is connected to SPI1 on the microprocessor. The nrf has the following specications which are useful in this project: Gaussian Frequency Shift Keying (GFSK) modulation, Manchester encoded 32 pins 8 SPI instructions for conguration Maximum transmit output power of 10 dbm (can be varied) Transmitted data rate 100 kb/s Can be used in the ISM bands 433, 868, or 915 MHz Carrier detection mechanism for "listen before talk" protocols Data Ready signal when a package is transmitted or received

16 INITIAL PROBLEM Address Match for incoming data detection Automatic retransmission Automatic Cyclic Redundancy Check (CRC) generation [8] The nrf also contains ve interval registers which is status, RF conguration, TX address, TX payload and RX payload. The TX address register has a length of four bytes which means that addressing of receivers should be done with one to four byte addresses. The address of transceiver itself is contained in the conguration register also four bytes wide. The payload registers is 32 bytes each which also determines the maximum package size. 1.3 Initial problem The hardware of the motes is being produced and assembled in parallel with this project by other people at Aalborg University meaning that no hardware will be developed in this project. The current motes does not have any sensing capabilities yet and this project will not aim for a specic measuring application. As the motes are brand new, no protocols have been developed or implemented for them. Thus the aim of this project is the development and implementation of a Medium Access Control (MAC) protocol for the ISM band module which makes it possible to establish direct and multi-hop connections between the motes.

17 Chapter 2 Analysis This chapter concerns issues to be considered when developing and working with wireless networks. When designing a protocol it is essential to be aware of which layers in network communication the protocol deals with. This project is working with the mechanisms of the data link layer which will be described here including classication and examples of dierent Multiple Access Protocols. The possible problems of wireless networks compared to regular wired networks will be investigated and the proposed mechanisms to solve them will be described. Energy consumption and real time aspects will briey be discussed as it is also important in wireless sensor networks. This analysis of wireless scenarios will lead to the choice of which type of MAC protocol to design and implement for the second version of the AAU mote. 2.1 The data link layer The data link layer is the second layer in the OSI reference model for network communication and is often referred to as layer two. It has interfaces to the physical layer and the network layer, 1 and 3 respectively. The layers of the OSI model and the location of the data link layer are shown in Figure 2.1.

18 THE DATA LINK LAYER 7 Application Application 6 Presentation Presentation 5 Session Session 4 Transport Transport Communication subnet boundary 3 Network Network Network Network 2 Data link Data link Data link Data link 1 Physical Physical Physical Physical Host A Router Router Host B Figure 2.1: The OSI reference model. The data link layer is layer 2 in the reference model and it deals with node-to-node rather than end-to-end communication [10]. The job of the data link layer is to provide an error free communication line to the network layer above. This is done at the sender by dividing the raw bit stream into data frames which are sent sequentially to the receiver over a wire-like channel, i.e. a channel that acts like a wire like a cable or a point-to-point wireless link. If the frame is received correctly, the receiver will send an acknowledgement frame back to the sender to inform him about it. The data link layer should also perform ow control of the transmission to prevent slow receivers from getting buer overow and thereby loosing data frames. This can be done by using feed back messages from the receiver allowing the sender to continue or slow down the transmission. As shown in Figure 2.1, data link layer protocols are operating between each machine in a network, i.e. routers or hosts which are interconnected. This is unlike layers 4-7 which features protocols dealing with end-to-end connections making the network and the machines within, transparent. The MAC layer is a sublayer of the data link layer i.e. it is not represented in the OSI model. This layer is used in networks where multiple machines need to communicate via a single communication channel. The protocols of the layer are called Multiple Access Protocols (MAP) and deals with the task of scheduling and determining which machine or node should have access to the channel next [10].

19 2.2. MULTIPLE ACCESS PROTOCOLS Multiple Access Protocols Starting in 1970 with the Aloha protocol, many algorithms for allocating a multiple access channel have been developed. This section will consider a classication of MAPs and use the Aloha protocol as an example of a simple way to share the used channel. Also carrier sensing is examined as a way of avoiding two nodes transmitting at the same time creating a collision. [5] Classication of Multiple Access Protocols At the highest level of the classication there are conict-free and contention protocols. The classication of MAPs is shown in Figure 2.2. [6] Dynamic Resolution Time of Arrival Contention Multiple Access Protocols Conflict Free Static Resolution Dynamic Allocation Static Allocation ID Probabilistic Probabilistic Reservation Token Passing Time and Freq Figure 2.2: Classication of Multiple Access Protocols [6] FIGURE 1.1: Classification of Multiple Access Protocols Freq. Based Time Based Conict free protocols are those scheduling the transmissions of all users [5]. In this way, by adjusting each user's transmitting time or frequency, it avoids that two or more users transmit simultaneously. Conict free transmission can be achieved by allocating the channel to the users either statically or dynamically. In the case of the static allocation, whether each user is active or not, the channel capacity is divided among the users and to each user assigned a x part. Hence the division can be done for a fraction of time like in Time Division Multiple Access (TDMA), where the channel capacity of one slot per frame is assigned to each user. The frequency bands division results in the Frequency Division Multiple Access (FDMA) protocol where a xed band is assigned to each user. The principles of FDMA and TDMA are shown in Figure 2.3 [6]. Section 1.1.: PROTOCOL CLASSIFICATION 3

20 MULTIPLE ACCESS PROTOCOLS Frequency Frequency Alocated time slot Alocated frequency band (a) Time (b) Time Figure 2.3: (a) The division of bandwidth in FDMA and (b) the division of time in TDMA The dynamic allocation assign a channel only to a user who has something to transmit. Thus, the user without transmitting data does not waste the channel capacity. This allocation can be further classied, based on the assignment scheduling, into reservation and token passing schemes. With reservation schemes, the users rst announce their intent to transmit and all those who have so announced will transmit before new users have a chance to announce their intent to transmit. With token passing schemes, a special frame (Token) is passed in order from one terminal to another terminal permitting only the token holder to transmit [6]. Contention protocol schemes dier from conict free schemes since there is no scheduling of transmissions. Hence, collisions may occur and the protocol should be able to solve those conicts when several users transmit simultaneously. Also the resolution process together with the idle users consume channel resources, which is a major dierence between various contention protocols. In order to guarantee a successful transmission, it is necessary to nd a way to avoid collisions. Also here a distinction between static and dynamic resolutions can be made. Static resolution means that the dynamics of the network does not have any inuence on the behavior of the system. The static resolution can be either probabilistic, meaning that the transmission of a packet happens with a xed probability, or based on ID, meaning that users have dierent priority in the network. The dynamic resolution can prioritize packets based on time of arrival or be probabilistic, but with a dynamic probability changing as a result of the interference in the network [6]. Both classes have advantages and disadvantages regarding resource usage, throughput and scalability. Some of these are listed in Table 2.1.

21 2.2. MULTIPLE ACCESS PROTOCOLS 19 MAP class Advantages Disadvantages Conict free No transmission interference Low throughput for each user Fair division of capacity Unused resources for idle users Contention Ecient for "bursty" users Resource consumption for error correction Ecient in ad-hoc networks Possible delay and unfair capacity division Aloha Table 2.1: Some of the advantages and disadvantages for the two classes of MAPs The Aloha protocol was used on Hawaii in the early 1970ties and was one of the rst design of a computer network via a shared medium (radio). The system was build on a hub/star typology and used two dierent frequencies where the hub broadcasted on the rst one and the clients were transmitting on the other frequency. The basic idea of this protocol is: If a client has a packet to send, it will transmit it. In case of collision in this transmission, the client will try to resend the packet later. This means that if the packet is successfully received by the hub, it immediately replies with the same packet as an acknowledgement. If the client never receives this reply the result is a collision and a retransmission must be made. The principle is called Pure Aloha and can be seen in Figure 2.4. Pure Aloha does only have a maximum throughput of about 18.4% due to collisions. Figure 2.4: An example of Pure Aloha with 2 client and a base station [4] Later this throughput was doubled to 36.8% by introducing the principle of Slotted Aloha. In slotted version of Aloha timeslots are introduced where a centralized clock transmits a tick in the beginning of each slot. The clients can only transmit when a tick is received (beginning of a new slot), this can be seen in Figure 2.5 [13].

22 MULTIPLE ACCESS PROTOCOLS Figure 2.5: An example of Slotted Aloha with 2 client and a base station [4] CSMA protocols An improvement to the Pure Aloha is to sense the carrier before accessing the medium. Protocols in which a node veries the absence of other trac before transmitting are called Carrier Sense Multiple Access (CSMA). Carrier Sense describes the fact that, before a node transmits, it "listens" to the medium to determine if another node in the neighborhood is transmitting on the same channel. If the medium is quiet, the node recognizes that this is an appropriate time to transmit. If a carrier is sensed, the node waits for the transmission in progress to nish before initiating its own transmission. In this way, the probability of a collision decreases.[7] Multiple Access describes the fact that multiple nodes send and receive on the medium. Transmissions by one node are generally received by all other nodes using the same medium. There are dierent variations of the CSMA protocol which is described below. 1-persistent CSMA When a station has data to send, it rst listens to the channel to see if anyone else is transmitting at that moment. If the channel is idle, the node transmits a packet immediately with a probability of 1. If the channel is busy, the node keeps listening and transmit immediately when the channel becomes idle. As soon as the channel becomes idle, all the nodes wishing to transmit access the medium at the same time. Collisions can occur only when more than one user begins transmitting within the period of propagation delay. Even if the propagation delay is zero, there will still be collisions because of the time from sensing the idle carrier to the transmission starts [10] [7]. Non-persistent CSMA To send data, a node rst listens to the channel to see if anyone else is transmitting and starts sending immediately if the medium is idle. If the medium is busy, the node waits a random amount of time and sense the channel again. Consequently, this algorithm leads to better channel utilization but longer delays than 1-persistent CSMA [10].

23 2.2. MULTIPLE ACCESS PROTOCOLS 21 P-persistent CSMA In p-persistent CSMA, the nodes also sense the medium before sending. If the channel is idle, transmit a packet with probability p and delay for one time slot with probability (1-p) and start over. If the channel is busy, then delay one time-slot and start over. Figure 2.6 shows the dierent states in p-persistent CSMA. [10] Idle Packet to send Sense the medium Medium Idle Medium busy Transmit decision Probability (p) Probability (1-p) Wait Send Figure 2.6: State diagram showing the principles of p-persistent CSMA. CSMA and Aloha comparison Figure 2.7 shows the computed throughput versus oered trac for all three x-persistent CSMA protocols, as well as for pure and slotted Aloha. In this gure the throughput S on the y-axis represents the expected number of successful transmissions per packet. The load G in the x- axis represents the number of attempted transmissions. Due to the possibility of collisions the load is usually bigger than the throughput. For example the throughput S for pure Aloha is S = Ge 2G and as seen on the gure it has a maximum value of S = 1/2e = when the load is equal to 0.5. The slotted Aloha instead has a throughput of S = Ge G. When the load is equal to 1, S has its maximum value of S = that is the one of pure Aloha. Figure 2.7 also shows how the CSMA protocols have a better throughput than Aloha protocols. [10]

24 another station has begun transmitting. In the latter case, the unlucky station acts as if there had been a collision (i.e., it waits a random time and starts again). If the station initially senses the channel busy, it waits until the next slot and applies the above algorithm. Figure 2.6 shows the computed throughput versus oered trac for all three protocols, as well as for pure and slotted 22 ALOHA MULTIPLE ACCESS IN WIRELESS NETWORKS Figure 2.7: Comparison of the channel utilization versus load for various random access protocols Figure [10]. 2.6: Comparison of the channel utilization versus load for various random access protocols[?] CSMA with Collision Detection In Carrier Sense Multiple Access With Collision Detection (CSMA/CD), if a collision occurs, the rst node which detects the collision sends a jam signal to all stations to indicate that there has been a collision. After receiving a jam signal, a node that was attempting to transmit abort its transmission and tries again later after waiting a random amount of time. The minimum time to detect the collision is the time it takes the signal to propagate from one station to the other and the maximum time needed is two times the propagation delay. This results in a much more ecient use of the media since the bandwidth of transmitting the entire frame is not wasted [7] Hidden terminal problem 2.3 Multiple access in wireless networks CSMA/CD scheme is a widely used MAC scheme for wired networks, but the use of this protocol in wireless networks results in additional problems. CSMA/CD is not really interested in collisions at the sender, but rather in those at the receiver. The signal should reach the receiver without collisions. But the sender is the one detecting collisions. The dierence here is in the signal strength, which remains almost the same for wired networks. For wireless networks, the signal strength decreases proportionally to the square of the distance to the transmitter. Obstacles in the line of sight attenuate the signal even further. This means that the collision at the receiver due to another sender, in many cases goes undetected at the sender. As the transmission power in the area of the transmitting antenna is much higher than the receiving power, collision detection is very dicult in wireless scenarios, and in practice not possible. There are several other issues to consider when moving from the wired domain into the wireless. Some of these are described in the following [7].

25 2.3. MULTIPLE ACCESS IN WIRELESS NETWORKS Hidden and Exposed terminal problems Figure 2.8 illustrates the hidden terminal problem. B is in the transmission range of A and C but C is not in transmission range of A and A is not in transmission range of C. Suppose that nodes A and C both want to transmit data to node B. They will both sense the medium free and transmit causing a collision at B. Hence, A is a hidden terminal for C and vice versa [7]. Figure 2.8: Node A and C are not in transmission range of each other. Thus they are hidden terminals to each other. Figure 2.9 illustrates the exposed terminal problem. B is in transmission range of A and C, and C is in transmission range of B and D. Suppose that node B is sending a packet to node A and C intends to transmit data to node D. C senses the medium to be busy and will not send any packet, postponing its transmission. In reality, no collision would have happened at A because A is outside the transmission range of C. Hence, this problem causes unnecessary delay. This means that C is exposed to B [7].

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